P.S. Actually I’m sympathetic to both arguments, even though they seem antithetical at first. Circling Squares says plasma is a sociological concept, while Harman says it’s a metaphysical concept.
In The Prince and the Wolf Latour gave primarily a sociological and epistemological explanation:
So plasma is what appears once the so-called natural sciences are added to the pot, so to speak, and made to circulate, not to cover the whole. (…) So, what people don’t understand is that when you do science studies you have completely different views of all that. The whole space is actually empty. And then in this very, very empty space where ignorance is the rule basically, you have circulating in the full vein, the very, very, very full vein, which is the circulation of active and formatted knowledge about mathematics, and about chemistry, and about physics, and about sociology, and about economics. So it is a reversal of background and foreground. Plasma is what you do when, to your shock, you make all of the formatted knowledge circulate inside the landscape. (p. 81)
Now, how do you call what is not formatted plasma? I mean, you can abandon the word if you want. But I think that’s the point with our criticisms: we are never in awe of or in dispute with the natural sciences. We like them because they occupy so little space! And when you’re struck by the ecological crisis, immediately you recognize a completely different territory. Here we know barely anything; we are in a state of complete ignorance. And then you have this very, very small channel of knowledge in the middle of a completely empty space. So suddenly you breathe (lots of space!) but then you are terrified by our shared ignorance, and then the question of reassembling the collective becomes central. (p. 82)
So if you take an organization (I’m very obsessed by the question of organization now). No organization would work one minute if it were not constantly drawing on this reserve of… so-called unformatted plasma. The point is just that we don’t know what it is exactly, of course. (p. 83)
So, plasma is completely… I mean it is a concept. If you want to show where the plasma is, I say everywhere
because it is… it’s not the unformatted that’s the difficulty here. It’s what is in between the formatting. Maybe this is not a very good metaphor. But it’s a very, very different landscape, once the background and foreground have been reversed and the sciences have been added to the landscape, instead of being what defined the landscape. (p. 84)
So this passage would seem to support Circling Squares’ argument. However, in Reassembling the Social, just after he first mentions “the strange figure of the ‘plasma’” (p. 50), Latour goes on to say
Most social scientists would adamantly resist the idea that they have to indulge in metaphysics to define the social. But such an attitude means nothing more than sticking to one metaphysics, usually a very poor one…” (p. 51).
He constantly argues for sociology to practice metaphysics and praises Tarde for doing so: “What is most useful for ANT is that Tarde does not make the social science break away from philosophy or even metaphysics” (p. 15). Harman therefore is also right to consider the concept of plasma within the metaphysics that Latour puts forward.
So is plasma a sociological or a metaphysical concept? I would say it’s both. This however doesn’t necessarily have to mean that it does work as such. Remember that Latour advocates the use of ‘weak terms’ as infra-language. So a concept like plasma is kind of a probe: it is sent forth as part of an experiment, the result of which can be either success or failure (and probably there is some zombie state in-between the two). My guess is that Latour probably wanted to use the concept as both sociological and metaphysical, but it is designed in such a way that if it fails as one (e.g. as a metaphysical concept when put under scrutiny by a philosopher like Harman), it can still carry on as a sociological concept. (After all Latour did say that “Maybe this is not a very good metaphor. ” ) I heard some people criticise this strategy as flip-flopping or being slippery, but it is consistent with Latour’s pragmatist commitments.